Pavement markings of various forms (e.g., paints, tapes, and individually mounted articles) are well known to guide and direct motorists as they travel along a highway. During the daytime, the markings, which are typically of selected distinctive color(s), may be sufficiently visible under ambient light to effectively signal and guide a motorist. At night, however, especially when the primary source of illumination is the motorist's vehicle headlights, the colors of the markings are generally insufficient to adequately guide a motorist. For this reason, pavement markings with retroreflective properties have been employed.
U.S. Pat. No. 4,072,403 (Eigenmann) discloses a retroreflective assembly useful in pavement marking applications that is particularly useful for situations in which retroreflection is required under raining and/or condensing environmental conditions. The assembly disclosed therein comprises a transparent globule having a first, uncoated partspherical surface portion positioned to be impinged by light rays, a second substantially flat surface portion opposite the first surface portion, and a third, toroidal surface portion intermediate between the first and second surface portions. The toroidal surface has a radius of curvature considerably smaller than that of the first part-spherical surface portion. A monolayer of transparent microspheres, with a reflective layer therebehind, disposed on the flat portion and at least a part of the toroidal surface portion of the globule serves to focus and retroreflect light beams that are incident to the first spherical surface portion of the globule and then pass through the globule to the toroidal or flat surfaces. The microspheres serve to redirect light back through the globule, exiting the upper surface so as to return towards the illumination source. The retroreflective assemblies disclosed in the patent, sometimes referred to as "globule/microsphere retroreflective assemblies", provide effective retroreflection of light at low incidence angles, making them well-suited for horizontal applications such as pavement surfaces.
U.S. Pat. No. 4,072,403 also discloses a method for production of such assemblies wherein a flat sheet material is passed through calendering and forming rolls to produce a web of globules with the spherical surface of the globules arising from one surface of the web and the flat and toroidal surfaces arising from the opposite side of the web. The resultant globules remain interconnected by a thin, breakable portion of the sheet. The position of the interconnection on individual globules is determined by the dimensions and spacing of the calendering and forming rolls. It is typically preferred that the interconnection be at about the boundary of the first, part-spherical surface portion and the third, toroidal portion of the globules. Production of the assembly further requires application of transparent binder material to the toroidal and flat side of the globules, followed by application of a monolayer of microspheres. Next, a very thin layer of transparent binder material is applied, e.g., by spraying, over the monolayer of microspheres. Throughout the remaining discussion, this second layer of transparent binder material is referred to as a "space coat", its function being to separate the reflective layer described below from the microspheres. Finally, a reflective layer is applied over the space coat, e.g., by spray metallization. The web may be then broken either into arrays of a plurality of assemblies or into individual assemblies, each comprising a single globule. The assemblies or arrays can be applied either to painted surfaces on the roadway or incorporated in pavement markings for subsequent application to a pavement surface. The Minnesota Mining and Manufacturing Company ("3M") distributes such assemblies under the trade designations GLOBEAD.TM. (single assembly in each unit) and GLOGUIDE.TM. Wet Reflective Elements (plurality of assemblies in each unit).
Retroreflective assemblies produced according to U.S. Pat. No. 4,072,403 have failed to exhibit the full potential retroreflective brightness which is theoretically expected from the optics of the assembly. Commercial production has resulted in unacceptably high amount of unsatisfactory assemblies that must be rejected as scrap. The webs from which globule/microsphere assemblies are formed are typically somewhat stiff and boardy and many vapor coating processes for application of aluminum reflector layers entail passing webs around rollers. As a result the web may crack during application of the metal reflective layer, yielding arrays of assemblies of irregular and undesired size. Furthermore, even when acceptable assemblies and arrays have been produced, they exhibit a pronounced directionality of efficiency of retroreflection. By "directionality" it is meant that the efficiency of retroreflection is significantly different in one orientation than another. In addition, the assemblies and arrays typically have a pronounced gray cast when viewed under daylight conditions. In addition to being aesthetically unappealing, this gray cast tends to detract from the conspicuity of pavement markings bearing such assemblies and arrays during daylight conditions.
Thus, there exists a need for an improved globule/microsphere retroreflective assembly of the type disclosed in U.S. Pat. No. 4,072,403 as well as an improved method for making such an assembly.